This invention relates in general to anti-lock brake systems and in particular a method and apparatus for testing the operation of a microprocessor arithmetic logic unit in an anti-lock brake system control module.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. An ABS further includes an electronic control module which is electrically connected to wheel speed sensors mounted adjacent to the controlled wheels, the solenoid valves and pump motor. The control module can be mounted directly upon the control valve or located remotely therefrom. The control module includes a microprocessor which is programmed to control the ABS in accordance with algorithms and parameters permanently stored in a read only memory (ROM). The microprocessor also can access a random access memory (RAM) for temporary storage and retrieval of data.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is responsive thereto to close a power relay. The power relay controls the supply of electric power to enable the solenoid valves and the pump motor. The microprocessor selectively actuates the solenoid valves in the control valve in accordance with algorithms in the ROM to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle.
Upon actuation, the ABS preempts control of the vehicle brakes from the vehicle operator. Accordingly, a typical ABS includes a number of diagnostic tests to assure that the ABS is functioning properly. Upon initial power up of the ABS, which occurs every time the ignition switch is actuated, the microprocessor performs a check sum on the control algorithm by cycling through all the instructions included in the algorithms stored in the ROM. The bits remaining in the microprocessor accumulator following each instruction are added to produce a cumulative total. The cumulative total is subtracted from an expected total. If the resulting difference is non-zero, the ABS is disabled and a warning light on the dash board illuminated to inform the vehicle operator that service is needed for the ABS.
Also, upon initial power up, the microprocessor will test the RAM by storing first a one, and then a zero in each bit in the RAM. After each entry, the microprocessor will interrogate each bit to verify that the correct entry is present. If a difference occurs, the ABS is disabled and the warning light illuminated.
During ABS braking cycles, when the power relay is closed, various electrical outputs are periodically tested to verify operation of the solenoid valves, pump motor and power relay. If the tests indicate a malfunction, an error code is generated which causes the ABS to be disabled and the warning light illuminated.